Semiconductor processing tools often use components called “showerheads” to distribute process gases, e.g., reactants, across a semiconductor substrate during processing. A typical showerhead may include a faceplate that forms one wall of an internal plenum; the faceplate may generally be oriented to be parallel to, and facing, a substrate being processed using the showerhead. The faceplate may have a large number of gas distribution holes arrayed across it, and gases introduced into the internal plenum may flow through the gas distribution holes and towards a substrate or substrates processed using the showerhead.